How do I model State, Let me count the ways by Ian Foster

We must sometimes define Web service interfaces that support operations on state, i.e., data values associated with a service that persist across operations, so that the result of one operation can depend on prior operations. We describe four different approaches to modeling state in Web service interfaces, based on the use of WSRF mechanisms, WS-Transfer mechanisms, distinct state identifiers, and simple HTTP, respectively. We show how each approach can be used to define an interface to a simple job management system, and summarize arguments that have been made for and against each approach.

http://www.gridforum.org/mail_archive/dais-wg/2005/05/doc00000.doc